When the Twin Towers came down 11 years ago (almost to the day), the world was struck numb. Even New Yorkers, who felt the trauma rumble through their veins, couldn’t get past the initial disbelief: how can this be happening? How can something so big, so invincible, actually be so vulnerable?

Hundreds of comments have been hurled at Renzo Piano’s “Shard,” the massive, reflective skyscraper that hulks over the London skyline – it’s big, no, huge; it’s out of the context of its Victorian neighborhood; its exclusive price tag could only be footed by Qatar royalty (as it is) – but few, beyond writing off the tower as a symbol of arrogance or hubris, have stopped to consider its impetus.

For that, we must look at the Shard in the context of 9/11. Only then can the Shard be understood for what it is: the amplification and perfection of the glass tower Piano began in post-9/11 New York, a utopian vision that stands defiantly in defense of the city itself.

When you consider the works of Renzo Piano, it would be difficult to discern a signature “style.” Which is fine by him (he hates the word). For Piano, architecture is about telling the story of the building he designs. And, as he puts it, “How can you tell such with the same language?”

There are, of course, themes that recur in Piano’s ouevre. Even from his first major work, the Centre Pompidou he designed with Richard Rogers, Piano was attuned to the sea-changes of the time: a distrust of the institution, a desire to break down the formalities of the past, a need for community space. Thus, the Pompidou became “a joyful urban machine” designed for human interaction.

And while you can certainly point to Piano’s obsession with craftsmanship – Piano comes from a family of builders after all – his experimental efforts with physical materials are almost always meant to explore their very un-physical potential for “flight.”

Lightness, transparency, vibrancy – these characteristics constantly present themselves in Piano’s work, and they have been fine-tuned over time. The Menil Collection, completed in the mid-80s, was Piano’s first triumph in light; ever since, his architecture, unlike any other, has somehow learned to “capture the sun.”

Thus when Piano begin his design for The New York Times building in New York, transparency and “contextuality” were two sides of the same coin. Piano, who was in New York the day of the terrorist attacks, designed a skyscraper with validity in a post-9/11 world: in comparison to the bulky opacity of the World Trade Center, Piano opted for a tower of light, of transparency, of permeability.

The New York Times architecture critic at the time, Nicolai Ouroussoff, who was working in the building as he wrote his critique, couldn’t hide his delight of it. Ouroussoff found it to be reminiscent of “a more comforting time,” a New York of the 50s and 60s, and, if I may extrapolate, of the 90s, when there was confidence in the marvelous structures that characterized the city.

The tower’s ground floors are open to the public, allowing people to pass through as they journey from 40th to 41st street; Ouroussoff marveled at the “continuous public performance:” “The flow recalls the dynamic energy of Grand Central Terminal’s Great Hall or the Rockefeller Center plaza, proud emblems of early-20th-century mobility.”

The only flaw that Ouroussoff noted was the tower’s rooftop – which, instead of “gathering momentum as it rises” and “dissolving into the sky,” as Piano intended, just “seems to fizzle.”

The building was an attempt to capture the workings of the city itself; Piano wanted the building to stand as an emblem, but at the same time dissolve into the bustle of the city streets. As he told BBC correspondent Razia Iqbal in a fascinating interview:

“There are no technical answers to terrorism – the only technical answer would be living in caverns, which is impossible. Which is why we built The New York Times building transparent and permeable, because transparency is safer than opacity. You cannot answer the risk of terrorism by giving up on the idea of a city being a great human invention, we have to defend it.”

The New York Times Building may have been the first incarnation of Piano’s desire to defend the city, but it was the Shard that would be its ultimate expression.

When Hal Foster, a Princeton University Professor, was asked his opinion of The Shard by The Guardian, he quipped, “It looks like a project from the 9/11 competition which grew frustrated with delays on the Hudson and moved to the Thames.” Indeed, each element that Piano considered in the post-9/11 context of the NYT Building is only intensified in the Shard.

The permeability of the New York Times building, which Piano engendered in order to encourage human interaction and movement (causing Ouroussoff to liken its lobby to Grand Central Station), is already inherent to the Shard, as it’s physically placed atop one of London’s most crowded Transportation Hubs. In fact, it was that location which justified the Shard’s existence in Piano’s mind: the Hub was “one of the few positions where you can have towers.”

Further vital to the Shard’s identity is its multi-functionality, which enhances its permeability further. Piano was insistent that the Shard be filled with restaurants and shops, so the public could use it at all hours of the day – not just until 6pm. Also key is a public viewing gallery, where the public could partake of the building’s staggering views.

Moreover, the glass, which gave The New York Times Tower its transparency, is different in the Shard. Rather than merging with its context, it responds to it; as the light changes, the facade reflects the river, the sky, the city. The pinnacle, which awkwardly “fizzled” in the New York tower, with the Shard, “dissolves into the sky,” to the point where, in Piano’s words, it “breathes in the clouds.”

Unlike the New York Times Building, which depends on the activity around it to give it life, The Shard is very much its own, living entity. Piano’s utopian monument to the city – dense, diverse, and multi-functional - of the 21st century.

Of course, as much as the Shard proclaims London’s position as a global city of the future, responding to the context of London as a whole, it comes at the expense of the neighborhood it calls home.

Despite Piano’s claim that the Transportation Hub justifies the Shard, there is no denying the modern skyscraper is completely disruptive to its Victorian surroundings. Nor can it be denied that the Shard would never have been allowed in a historical neighborhood whose residents had money and power – unlike in Southwark, one of London’s poorer boroughs.

The Shard, which is currently the pinnacle of Renzo Piano’s long, inspired career, has stirred up controversy for its size, its location, its ownership. But it is also a reflection of what a skyscraper, if it is to exist at all, must be in a post-9/11 world.

At that same BBC Interview, a Londoner walked up to the microphone to ask Mr. Piano a question: “I think since 9/11 we’ve been worried a little bit, I have anyway, about iconic buildings. Are you worried that it’s throwing out a bit of a defiant message, towering over London the way it does?”

For Piano, that defiance was the point – but it was also a gamble. Indeed, to some, this glass tower will always be an offensive re-incarnation of the arrogance and power of our pre-9/11 world; but, in the masterful hands of Piano, it’s a quietly intense defense of the city itself.